This invention relates to a gear selection method for an electronically controlled motor vehicle transmission, and more particularly to a method that enhances the effectiveness of vehicle rocking maneuvers.
When the forward or reverse momentum of a vehicle is lost on snow or soft ground to the point that it cannot be driven further, it may still be possible for the driver to recover sufficient momentum to continue driving by alternately shifting the transmission between forward and reverse speed ranges to rock the vehicle back and forth. In a vehicle equipped with a manual transmission, the driver is typically advised to shift between reverse and the first (highest numerical input/output ratio) forward gear for optimum rocking effectiveness. However, when the vehicle is equipped with an automatic transmission and the driver simply shifts between Reverse and Drive, the optimum forward gear is not always engaged, particularly if the wheel slip is relatively high. This occurs because forward gear selection in an automatic transmission is ordinarily determined as a function of the transmission output speed, which presumably corresponds to the vehicle speed. However, in rocking maneuvers, the transmission output speed is influenced more by wheel slip than vehicle speed, and the transmission controller will often schedule an upshift to the second or third forward gear based on the apparent vehicle speed. In some cases, the controller may even engage the second forward gear as soon as the driver shifts to Drive. In both cases, the effectiveness of the rocking maneuver is significantly degraded, and the driver may be unable to regain sufficient vehicle momentum to continue driving.
Although a transmission controller could theoretically be programmed detect the presence of a rocking maneuver, and to limit the forward speed range to the first gear during such maneuvers, it is difficult in practice to distinguish rocking maneuvers from other maneuvers for which forward range upshifting may be appropriate. Accordingly, what is needed is a transmission gear selection method that enhances the effectiveness of rocking maneuvers without preventing upshifting during normal vehicle operation.
The present invention is directed to an improved method of operation for a motor vehicle automatic transmission in which the vehicle speed for purposes of selecting a desired transmission gear is determined at least in part based on the wheel speeds of the vehicle. If the vehicle is being operated in a two-wheel drive mode, the vehicle speed is determined based on an average of the speeds of the un-driven wheels; if the vehicle is being operated in a four-wheel drive mode, the vehicle speed is determined based on the lowest of the four wheel speeds. Optionally, the foregoing methodology is only enabled if the Reverse range of the transmission has been engaged within a specified time interval, and otherwise the vehicle speed is determined based on an output speed of the transmission.